1. Field of the Invention
The present invention relates generally to a process for preparing resinous pre-ceramic materials, also known as superconductive precursors, which can be converted into electrically superconducting ceramic articles having pervoskite structure and the formula ABa.sub.2 Cu.sub.3 O.sub.7-x, where A is a rare earth metal and x is from 0.5 to 0. Wires, fibers, films, ribbons, as well as bulk articles can be formed from the pre-ceramic material and converted into superconductive articles. More particularly, this invention relates to the preparation of resinous pre-ceramic materials which are formed from homogenous solutions and which are soluble in nonpolar alcohol solvent solutions. The use of two solvent systems allows for controllable viscosity of the pre-ceramic material and the ability to more easily form articles such as wires, ribbons, fibers, or bulk material which can then be heat treated in a dry oxygen atmosphere to yield superconductive properties.
2. Background of the Invention
The recent discovery of superconductive ceramics which exhibit superconductivity above 77.degree. K (liquid nitrogen boiling temperature) has generated a tremendous search for commercial applications. Particularly promising are the perovskite structured superconductors having the formula ABa.sub.2 Cu.sub.3 O.sub.7-x , where A is a rare earth metal and x is from 0.5 to 0. These copper-based superconductors have been found to have a relatively high superconducting transition temperature (T.sub.c). This is the temperature at which a material leaves its normal conductivity state and exhibits little or no resistance to electric current. The copper-based superconductors have been known to carry a critical current in excess of 10.sup.5 amp/cm.sup.2 at liquid nitrogen temperature (77.degree. K).
Numerous difficulties have been encountered, however, in achieving a practical usefulness. In order to be truly useful, the superconducting material must be capable of being fabricated into specific shapes, such as wires, fibers, films, coatings and bulk articles. If these materials can be successfully manipulated to achieve a desired article or shape, applications such as microelectronic circuitry, transmission wires, power generators, magnetically levitated trains, magnetic containment fields for storing electricity as well as well as various medical applications become possible.
Various other factors such as proper ceramic density, sufficient capability to handle large currents, good mechanical strength and flexibility also play an important part in making a commercially viable superconducting material.
Thus far, several methods have been used to produce copper-based superconducting ceramics. Solid-state sintering was the first method of producing superconducting materials. This method required the powder oxides and carbonates of the various constituents to be mixed and reacted at high temperatures, followed by a succession of regrinding and refiring steps. Although the solid-state reaction method is found to be very convenient to form a superconductor material, X-ray diffraction analysis shows that these superconductors are usually multiphased, i.e. contain other unreacted compounds such as BaCuO.sub.2 and CuO. More recently, superconducting ceramics have been prepared by coprecipitation methods using the nitrate and/or other inorganic salt forms of the individual constituents and precipitated out of solution to the corresponding hydroxide/oxide containing carbonate forms. The precipitated mass is heated to remove water and anions impurities and then heated and ground repeatedly in the same manner as the solid-state reaction method. The advantage of this method over the solid-state method is that if the stoichiometry of the desired single phase of the superconductor is known, as it is for YBa.sub.2 Cu.sub.3 O.sub.7-x (the composition 1-2-3 compound), then the added chemical step ensures that the constituents will be formed and that other "impurity" phases will not be formed.
Two other known forms of the coprecipitation method have been tried, namely the citrate and oxalate methods. In each case, the nitrates of the constituent powders are first dissolved in solution. Then, in the citrate method, citric acid and ethylene glycol are used to initiate the precipitation instead of Na.sub.2 CO.sub.3 or K.sub.2 CO.sub.3. In the oxalate method, potassium oxalate is used. These methods must be monitored closely and in the oxalate method the pH must be strictly monitored to avoid formation of double salts.
The product of both the solid state processes as well as the coprecipitation methods is typically a powder or sintered spongy compact, which are difficult to manufacture into applications such as wires, fibers, films or coatings.
A solution process has been used for fabricating superconducting ceramics. This process comprised the steps of preparing a solution containing organometallic precursor molecules wherein the relative molar ratios of said organometallic precursor molecules are in the appropriate amounts for producing electrically superconducting ceramic material; treating said mixture of organometallic precursor molecules to form a viscous dielectric material; shaping said viscous dielectric material into a particular shape; and heating the shaped viscous material for a sufficient time and at a sufficient temperature to convert said viscous material into an electrically superconducting ceramic article.
This example also discloses the use of a sol-gel process to fabricate YBa.sub.2 Cu.sub.3 O.sub.7-x superconducting fibers. This process involves controlling the hydrolysis and polymerization of metal alkoxides to form primarily chain-like metaloxane polymers which are then shaped into desired fibers and heat treated to form a superconducting material. The fibers produced from this process showed the superconducting transition temperature to be about 90.degree. K. This application also discloses the use of 2-ethyl-hexanoic acid (2-EHA) as an organic acid modifier which reduces the hydrolysis and polymerization rates of the metal alkoxides, thus preventing precipitation. Once the yttrium, barium and copper alkoxide solutions were fully reacted, the solvent was removed and the solutions became viscous after a few hours due to the hydrolysis and polymerization of the alkoxides.
Up to the present time, however, no method has disclosed the use of a dual solvent system for the preparation of superconducting materials. The advantages of such a system allows for enhanced control over the formation of a viscous pre-ceramic which can be shaped and heat treated to form the superconductive state.